Prior efforts to achieve stabilization of a two spool turbofan engine have been accomplished by stabilization of the higher speed spool using high speed spool RPM as the controlling parameter. With the existence of the high by-pass ratio two spool engines, the low speed or fan spool RPM provides a parameter more closely related to engine thrust than the high speed spool RPM. Since requested changes in engine operation are usually indicative of desired changes in engine thrust, it becomes more advantageous to utilize the fan spool RPM as the controlling parameter to accomplish engine stabilization. However, because the inertia of the spool containing the fan is generally larger than the inertia of the high speed spool, it is more difficult to stabilize the fan spool RPM than to stabilize the high speed spool RPM.
To realize fan spool stabilization, current systems employ a control loop based upon an error signal proportional to the difference between the desired fan spool RPM and the actual fan spool RPM. Such error signal is then applied to a compensation network which in turn adjusts the fuel flow to the engine in such a manner as to minimize this error signal. These compensation networks generally consist of electronic components with gains and time constants that vary as a function of engine operating conditions.
The present invention is an improvement in two spool turbofan engine stabilization since the invention achieves fan spool RPM control with greater stability than that previously possible. Additionally, with this improvement, the fan spool may have sufficient stability such that the variable compensation networks may not be needed to maintain the desired spool stability.